The three basic components to any optical fibre communications based system are:
i. a transmitter, which converts an electrical signal to be transmitted into an optical form, PA0 ii. the optical fibre which acts as a waveguide for the transmitted optical signal, and the PA0 iii. optical receiver which first detects the optical signal transmitted and converts it to electrical form.
A typical optical receiver comprises a photodetector, e.g. a reverse biassed avalanche photodiode (APD) or PIN diode, coupled across the inputs of a high impedance amplifier, commonly a transimpedance amplifier. A feedback circuit from the amplifier output is commonly used to provide automatic gain control of the bias applied to the photodetector.
In practice optical receivers have to deal with very large variation of signal strengths. When strong optical signals are detected they usually lead to overloading of the frontend amplifier. The range between the maximum sensitivity and minimum overload point of an optical receiver is called the dynamic range of the receiver.
The present invention seeks to provide a means for increasing the dynamic range of an optical receiver by removing the overload criteria within the operating range without affecting the sensitivity of the receiver.
Examination of the time averaged D.C. photocurrent generated from the received optical signal shows that a linear relationship exists. However, when the optical power received is expressed in dBm relative to the photocurrent generated, an exponential curve is obtained, as shown in FIG. 1.
This curve is fundamental to an understanding of the overload problem since it clearly indicates the kind of current variation the frontend amplifier has to deal with at its input.
Examination of the characteristics of a semiconductor diode shows that the forward bias current variation through the diode with respect to the voltage developed across it is also nonlinear and follows an exponential form, as shown in FIG. 2.